Mode mixing optical fibers and methods and systems using the same

ABSTRACT

The present disclosure relates more to mode mixing optical fibers useful, for example in providing optical fiber laser outputs having a desired beam product parameter and beam profile. In one aspect, the disclosure provides a mode mixing optical fiber for delivering optical radiation having a wavelength, the mode mixing optical fiber having an input end, an output end, a centerline and a refractive index profile, the mode mixing optical fiber comprising: an innermost core, the innermost core having a refractive index profile; and a cladding disposed about the innermost core, wherein the mode mixing optical fiber has at least five modes at the wavelength, and wherein the mode mixing optical fiber is configured to distribute a fraction of the light input at its input end from its lower-order modes to its higher-order modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application of InternationalPatent Application no. PCT/US2016/046931, filed Aug. 12, 2016.International Patent Application no. PCT/US2016/046931 claims thebenefit of priority of U.S. Provisional Patent Application No.62/204,900, filed Aug. 13, 2015, which is hereby incorporated herein byreference in its entirety.

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

The present disclosure relates generally to optical fibers and opticalfiber lasers. The present disclosure relates more particularly to modemixing optical fibers useful, for example in providing optical fiberlaser outputs having a desired beam product parameter and beam profile.

2. Technical Background

High power optical lasers and amplifiers are widely used in a variety ofindustries for a variety of purposes, such as laser cutting, welding andmachining of various materials. Research and development in rare-earthdoped optical fibers along with the discovery of specialty fiber designssuch as Large-Mode Area (LMA) fibers has triggered the introduction of avariety of high power fiber laser and amplifier modules. Multi-kW fiberlasers and amplifiers have been realized with very high efficiencies andare fueling the growth of laser material processing. Of course, othertypes of high power lasers, such as solid-state lasers, are alsocommonly used in materials processing applications.

Lasers and amplifiers used in the field of materials processingdesirably fulfill specific requirements in terms of output power andbeam profile. In terms of power, the laser or amplifier system desirablydelivers radiation with a wavelength and an energy that is high enoughto process a desired material, typically on the order of kilowatts. Twosorts of kW-level fiber lasers can be distinguished: multi-mode andsingle-mode. Single-mode fiber lasers typically deliver on the order of1-3 kW of optical power, while multi-mode fiber lasers typically operatein the range of several tens of kW of output power. For materialprocessing applications, both single mode and multi-mode fiber lasersare used. A multi-mode laser can be configured, for example, by using amulti-mode active fiber, or by combining the outputs of several singlemode fiber lasers into a multi-mode delivery fiber for delivery to aworkpiece. Similarly, a multi-mode delivery fiber is often used todeliver power from a solid-state laser to a workpiece.

In terms of beam profile, users typically desire the delivered beam tohave a desired Beam Parameter Product (BPP). As used herein, the BPP isdefined as the product of the beam radius R and the divergence angle ofthe beam θ, expressed in units of mm·mrad. The beam radius R in mm isdefined as half of the Beam Diameter measured at 13.5% of the maximumintensity as the beam emerges from the optical fiber. The divergenceangle θ in mrad is defined as the half-angle formed with the opticalaxis as the beam propagates from the end of a beam delivery opticalfiber. While desired BPP values will vary from application toapplication, three typical ranges of BPP values for fiber-coupled lasersare provided below:

-   -   1.5 to 2 mm·mrad for a 50 μm core diameter beam delivery cable    -   3 to 4 mm·mrad for a 100 μm core diameter beam delivery cable    -   6 to 8 mm·mrad for a 200 μm core diameter beam delivery cable

Moreover, in many applications, the delivered beam has an intensityprofile that is substantially evenly distributed along the beam. Such a“flat-top” profile is different from a Gaussian profile, in which themaximum intensity is only at the center with a relatively sharp drop-offin intensity away from the center. A “flat-top” profile can help toenable controlled and accurate cutting, welding or machining process.

In many applications, a beam with a substantially circular profile isalso (or alternatively) desired.

In order to use such lasers for material processing applications whilesatisfying the required beam parameter product (BPP), conventionaloptical fiber laser and amplifier systems have a single mode ormulti-mode laser or amplifier output coupled into a beam delivery cablefor transmission of the output to a workpiece. Similarly, conventionalsolid-state lasers are coupled to a beam delivery cable for transmissionof the laser output to a workpiece. Commonly used beam delivery cablesare made with highly multi-mode step-index fibers with typical corediameters of 50, 100, 200, 400 and 600 microns and numerical apertures(NA) varying from 0.1 to 0.4 (and often greater than 0.4). A number oftechniques have been attempted to provide both a desired BPP and adesired flat-top profile, such as offset splicing between a single modelaser output (launch fiber) and the beam delivery cable, beam deliveryoptical fibers with shaped cores, external beam shaping techniques,mechanical fiber micro-bending, fiber tapers (adiabatic and/or abrupt),long period gratings and multimode interference in multi-mode fibers.However, each of these suffers from a number of drawbacks.

Accordingly, there remains a need for improved optical fibers, systemsand methods that can, for example, provide one or more of a desired BPPvalue, a desired intensity profile (e.g., a “flat top” intensityprofile), and a circular beam shape.

SUMMARY OF THE DISCLOSURE

In one aspect, the disclosure provides a mode mixing optical fiber fordelivering optical radiation having a wavelength, the mode mixingoptical fiber having a input end, an output end, a centerline and arefractive index profile. The mode mixing optical fiber includes

an innermost core, the innermost core having a refractive index profile;and

a cladding disposed about the innermost core,

-   wherein the mode mixing optical fiber has at least five modes at the    wavelength, and-   wherein the mode mixing optical fiber is configured to distribute a    fraction of the light input at its input end from its lower-order    modes to its higher-order modes.

In another aspect, the disclosure provides an optical system comprising:

-   -   a mode mixing optical fiber as described herein; and    -   a first optical fiber having an output end directly optically        coupled to the input end of the mode mixing optical fiber, the        first optical fiber being configured to propagate optical        radiation having the wavelength.

In another aspect, the disclosure provides an optical system comprising:

-   -   a mode mixing optical fiber as described herein; and    -   an optical source (e.g., a solid state laser) optically coupled        to the input end of the first mode mixing optical fiber.

In another aspect, the disclosure provides a method for providing guidedradiation of the wavelength having a desired intensity profile. Themethod includes coupling input radiation into a first end of a modemixing fiber as described herein, and guiding the radiation along themode mixing optical fiber to provide guided radiation having a desiredintensity profile, e.g., a flat-top intensity profile as describedherein.

In another aspect, the disclosure provides a method for providing a freespace-propagating optical beam, the method comprising

-   -   providing an optical system as described herein;    -   propagating radiation of the wavelength into the mode mixing        optical fiber; and    -   propagating the free space-propagating optical beam from the        output end of the mode mixing optical fiber.

These as well as other aspects, embodiments, advantages, andalternatives, will become apparent to those of ordinary skill in the artby reading the following detailed description, with reference whereappropriate to the accompanying figures. Various embodiments of theoptical fibers, systems and methods described herein can be useful inlaser machining applications as well as in a variety of additionalapplications that would benefit from, for example, fiber beam controltechniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view and FIG. 2 is a schematicside view of a mode mixing optical fiber according to one embodiment ofthe disclosure.

FIG. 3 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 4 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 5 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 6 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 7 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 8 is a cross-sectional schematic view of a mode mixing opticalfiber according to another embodiment of the disclosure.

FIG. 9 is a schematic side view of a mode mixing optical fiber accordingto another embodiment of the disclosure.

FIG. 10 is a schematic view of an optical system according to oneembodiment of the disclosure.

FIG. 11 is a schematic view of an optical system according to anotherembodiment of the disclosure.

FIG. 12 is a schematic view of an optical system according to anotherembodiment of the disclosure.

FIG. 13 is a schematic cross-sectional view of the mode mixing opticalfiber used in the experiments of Example 1.

FIG. 14 is a picture of a cleaved fiber endface of the mode mixingoptical fiber used in the experiments of Example 1.

FIG. 15 is a graph showing the calculated power distribution among modesexcited in the mode mixing beam delivery cable in the experiments ofExample 1.

FIG. 16 is a 2D plot of the calculated total output intensity deliveredby the mode mixing beam delivery cable in the experiments of Example 1.

FIG. 17 is a plot of the calculated profile of the beam delivered by themode mixing beam delivery cable in the experiments of Example 1.

FIG. 18 is a graph showing the calculated power distribution among modesexcited in a conventional beam delivery cable in the experiments ofExample 1.

FIG. 19 is a 2D plot of the calculated total output intensity deliveredby a conventional beam delivery cable in the experiments of Example 1.

FIG. 20 is a plot of the calculated profile of the beam delivered by aconventional beam delivery cable in the experiments of Example 1.

FIG. 21 is a schematic view of a conventional system and a 2D graph ofthe total output intensity delivered thereby as described in theexperiments of Example 1.

FIG. 22 is a schematic view of an optical system including an offsetcore mode mixing fiber and a 2D graph of the total output intensitydelivered thereby as described in the experiments of Example 1.

FIG. 23 is a cross-sectional schematic view of the design of themode-mixing optical fiber used in the experiments of Example 2.

FIG. 24 is an index-contrast image of a cleaved endface of the fiber ofFIG. 22.

FIG. 25 is a graph showing the calculated power distribution among modesexcited in the mode mixing beam delivery cable in the experiments ofExample 2.

FIG. 26 is a 2D plot of the calculated total output intensity deliveredby the mode mixing beam delivery cable in the experiments of Example 2.

FIG. 27 is a plot of the calculated profile of the beam delivered by themode mixing beam delivery cable in the experiments of Example 2.

FIG. 28 is a schematic view of an optical system including an offsetlow-index ring mode mixing fiber used in the experiments of Example 2.

FIG. 29 is a 2D graph of the total output intensity delivered by thesystem of FIG. 27 as described in the experiments of Example 2.

FIG. 30 is an intensity profile of the output delivered by the system ofFIG. 27 as described in the experiments of Example 2.

FIG. 31 is a 2D graph of the total output intensity delivered in theexperiments of Example 3.

FIG. 32 is an intensity profile of the output delivered in theexperiments of Example 3.

FIG. 33 is a set of images providing the results of a first experimentdescribed in Example 4.

FIG. 34 is a set of images providing the results of a second experimentdescribed in Example 4.

FIG. 35 is a plot of beam divergence vs. fiber length for the modemixing fibers described in Example 5

FIG. 36 is a set of images providing the results of the experimentsdescribed in Example 6.

As the person of skill in the art will appreciate, the drawings are notnecessarily drawn to scale, and various elements of the system may incertain drawings be omitted for purposes of clarity.

DETAILED DESCRIPTION

In the following discussion it is assumed that the reader has the basicknowledge of the structure of optical fibers familiar to the person ofskill in the art. Thus, the concepts of a fiber core, cladding, andcoating are not discussed in detail. As is familiar to the person ofskill in the art, radiation having a wavelength propagates generally inthe core of the fiber, the diameter of which is typically in the rangeof a few microns to a several hundred microns, even in some embodimentsup to 1500 microns. The refractive index difference between the core andthe cladding acts to confine the light in one or more propagating modes,generally in the core of the fiber (although the person of ordinaryskill in the art will appreciate that some energy is actually present inthe cladding in the region near the core).

The terms “light” or “optical”, as used herein, are used broadly asunderstood by one of ordinary skill in the art of optical waveguides,and are not to be limited as pertaining only to the visible range ofwavelengths. Refractive indices described herein are described withreference to the wavelength of the radiation. In certain embodiments ofthe optical fibers, systems and methods described herein, the wavelengthis in the visible or near-infrared (e.g., in the range of about 0.5 μmto about 3 μm).

The inventors have noted that in a multi-mode beam delivery fiber, ifthe light is uniformly distributed among all available modes (i.e., allavailable transverse modes), the output beam can exhibit a relativelyflat-top intensity profile. Populating higher-order modes also affectsthe divergence angle of the output beam (since higher-order modespropagate at larger divergence angles) and increases the BPP. When allmodes are evenly populated, the beam divergence equals the fiber corenumerical aperture.

However, in conventional systems, all modes are not evenly populated.Rather, the number of modes excited in a multi-mode beam delivery fiberand the relative amount of power coupled in each mode is determined by,e.g., the spatial overlap between the incident laser radiation (e.g.,from an active fiber) and the modes of the core of the beam deliveryfiber. Since transverse modes are orthogonal, only the modes with anon-zero spatial overlap with the input laser beam (e.g., from an activefiber) can be populated. The relative amount of power carried by eachmode is determined by the fraction of spatial overlap. As a result, thebeam profile and the BPP available at the output of the beam deliverywill vary based on the particular type of laser or amplifier being used(i.e., depending on the profile of the laser/amplifier output). Forexample, when using a single mode laser or amplifier source, thesignificant difference in size and form factor with the multi-mode stepindex fiber in a conventional beam delivery cables results in a lowlevel of mode mixing (i.e., only a few lower order modes are typicallypopulated). Use of a multi-mode laser or amplifier fiber can helpmatters somewhat, but due to the fact that such multi-mode laser oramplifier fibers are typically only few-moded, the beam delivery fiberstill typically propagates radiation in only its lower order modes. Whenonly lower order modes are populated, the delivered beam is typicallymuch higher in intensity at its center than at its periphery. Whencoupling a solid-state laser to a beam delivery fiber, for example,through free-space optics, the beam delivery fiber can similarlytransmit radiation chiefly in its lower order modes, similarly leadingto a delivered beam having a more intense center.

The present inventors have addressed the drawbacks in the prior art byproviding optical fibers configured to, for example, couplelaser/amplifier radiation (e.g., from single-moded or few-moded fiber,or coupled from a solid-state laser) input at its input end (andpropagating toward its output end) into its higher order modes. Such anoptical fiber, when used as a mode transformation fiber or a beamdelivery fiber in an optical fiber laser or amplifier system can providean output having one or more of a desired BPP value, a desired intensityprofile (e.g., a “flat top” intensity profile), and a circular beamshape. In certain embodiments, such mode mixing optical fibers can beprovided by introducing asymmetry within the core in order to perturbthe mode overlap between an input optical fiber and the mode mixingoptical fiber, thereby increasing mode mixing.

As the person of ordinary skill in the art will appreciate, the opticalfiber designs described herein are scalable, and offer many degrees offreedom to fulfill the needs of the end-user in terms of BPP whilemaintaining a desired intensity profile (e.g., a “flat top” and/orcircular beam). Based on the present disclosure, the person of ordinaryskill in the art can use conventional optical simulation techniques toprovide additional designs within the scope of the disclosure.

Advantageously, such systems can be provided in an all-fiber monolithicconfiguration using standard fusion splicing procedures and conventionalcommercial splicing equipment. Such an all-fiber approach can offer easyand simple handling, implementation and maintenance. As the person ofordinary skill in the art will appreciate, the optical fibers, methodsand systems described herein do not require external elements, spatialfiltering or special treatment to be operated and to perform modeup-conversion. The optical fibers described herein can be packaged intobeam delivery cables and simply spliced to the output of the laser, andthus are compatible with existing optical fiber laser and amplifiersystems. Similarly, the optical fibers described herein can be coupled,for example through free-space optics, to the output of other types oflasers, such as solid-state lasers.

An embodiment of the disclosure is shown in cross-sectional schematicview in FIG. 1, and in schematic side view in FIG. 2. Mode mixingoptical fiber 100 has an input end 102 and an output end 103. Modemixing optical fiber 100 also has a centerline 104 (defined as the pointat the geometrical center of the cross-section of an optical fiber), anda refractive index profile (defined as the refractive index as afunction of position of the cross-section of an optical fiber). The modemixing optical fiber 100 includes an innermost core 110 (which has itsown refractive index profile, defined as the refractive index as afunction of position of the cross-section of the innermost core of anoptical fiber); and a cladding 120 disposed about the innermost core.The mode mixing optical fiber is configured to deliver optical radiationhaving a wavelength (i.e., from its input end to its output end).Notably, the mode mixing optical fiber has at least five modes (i.e.,modes substantially confined by the innermost core) at the wavelength.For example, in certain embodiments, the mode mixing optical fiber hasat least seven modes at the wavelength, or at least ten modes at thewavelength. In other embodiments, the mode mixing optical fiber has atleast twenty, at least thirty, at least forty or even at least fiftymodes at the wavelength.

Critically, the mode mixing optical fiber is configured to distribute afraction of the light input at its input end (and propagating toward itsoutput end) from its lower-order modes to its higher-order modes.Through the distribution of optical power among not only the low-ordermodes but also the higher-order modes of the mode mixing optical fiber,a beam can be output from the output end of the mode mixing opticalfiber that has, for example, a desired BPP and/or beam shape, such as asubstantially “flat-top” profile. The present disclosure identifies anumber of ways to configure a mode mixing optical fiber such that itdistributes a fraction of the light input at its input end from itslower-order modes to its higher-order modes. For example, in certainembodiments of the disclosure, the innermost core of the mode mixingoptical fiber has a centerline (i.e., defined as described above, butwith reference to the innermost core as opposed to the overall fiber)that is positioned substantially non-collinearly with the centerline ofthe optical fiber. In other words, in certain embodiments, the innermostcore of the mode mixing optical fiber is disposed off-center withrespect to the overall mode mixing optical fiber. FIG. 3 is across-sectional schematic view of a mode mixing fiber 300, which has aninnermost core 310 and a cladding 320 disposed around the innermostcore. In this embodiment, innermost core 310 has a centerline 314 thatis disposed substantially off-center with respect to the overall fiber300. That is, the center of the innermost core 310 is laterally offsetfrom the centerline 304 of the overall fiber 300. The lateral offset ofthe center of the innermost core with respect to the centerline of thefiber is at least 5 microns, for example, at least 10 microns, at least20 microns, or at least 30 microns. In this embodiment, the mode mixingoptical fiber has a step index profile; the person of ordinary skill inthe art will appreciate that other index profiles may be used.

The person of ordinary skill in the art will appreciate that theinnermost core of the mode mixing optical fiber can take a variety ofshapes. For example, in certain embodiments, as shown in FIG. 3, theinnermost core has a substantially circular cross-sectional shape. Whenthe mode mixing optical fiber has an innermost core that issubstantially circular in cross-sectional shape, it desirably includessome other feature or characteristic that can provide for the desireddistribution of radiation among modes. For example, it can have alaterally-offset innermost core, as described above. In otherembodiments, the innermost core can have a refractive index profileconfigured to distribute radiation among modes, as described in moredetail below.

In other embodiments, the innermost core of the mode mixing opticalfiber has a substantially non-circular cross-sectional shape. Forexample, mode mixing optical fiber 400, shown in cross-sectionalschematic view in FIG. 4, has an innermost core 410 that has asubstantially rectangular shape (here, a square). In certainembodiments, the substantially non-circular innermost core is centeredalong the centerline of the mode mixing optical fiber (i.e., theinnermost core has a centerline that is positioned substantiallycollinear with the centerline of the optical fiber). But in otherembodiments, the substantially-non circular innermost core is laterallyoffset from the centerline of the optical fiber, e.g., in any manner asdescribed above with respect to FIG. 3. A variety of other substantiallynon-circular shapes can be used for the innermost core in suchembodiments. For example, the innermost core can have a polygonal shape(e.g., a regular polygon or an irregular polygon), with any desirablenumber of sides (e.g., triangular, rectangular, pentagonal, hexagonal,octagonal). The vertices of the polygon can be sharp or somewhatrounded. Of course, a substantially non-circular innermost core need notbe polygonal; it can have a rounded but non-circular shape (e.g., oval,elliptical, semicircular, etc.).

In certain embodiments, the innermost core of the mode mixing opticalfiber has one or more substantially up-doped regions and/or one or moresubstantially down-doped regions, configured to provide the desireddistribution of radiation among its propagating modes. As will bedescribed in more detail below, the one or more substantially up-dopedregions and/or one or more substantially down-doped regions can beconfigured in a number of ways. An up-doped region is a region that hasa higher refractive index at the wavelength than the remainder of theinnermost core. The person of ordinary skill in the art will appreciatethat this can be due to the up-doped region having more of a highrefractive index dopant, or less of a low refractive index dopant thanthe remainder of the innermost core. A down-doped region is a regionthat has a lower refractive index at the wavelength than the remainderof the innermost core. The person of ordinary skill in the art willappreciate that this can be due to the down-doped region having less ofa high refractive index dopant, or more of a high refractive indexdopant than the remainder of the innermost core. The person of ordinaryskill in the art will appreciate that an up-doped or down-doped regionneed include any dopant at all, e.g., it can be substantially puresilica, in cases in which the remainder of the innermost material has adifferent refractive index difference.

In certain embodiments, the innermost core of the optical fiber includesone or more substantially down-doped regions disposed symmetricallyaround its centerline. For example, the innermost core of the modemixing optical fiber can include a substantially down-doped regionhaving a centerline disposed substantially collinear with the centerlineof the innermost core. One such embodiment is shown in cross-sectionalschematic view in FIG. 5. Mode mixing optical fiber 500 has an innermostcore 510 surrounded by a cladding 520. Innermost core 510 includes asubstantially down-doped region 515, which has a centerline disposedalong the centerline of the innermost core itself. Of course, in otherembodiments there is no substantially down-doped region disposedsymmetrically around the centerline of the innermost core.

In certain embodiments, the innermost core of the mode mixing opticalfiber includes one or more substantially down-doped regions that aredisposed asymmetrically around the centerline of the innermost core.Such substantially down-doped regions may be provided, for example, incombination with a down-doped region provided along the centerline ofthe innermost core as described above, or may be provided in the absenceof a down-doped region provided along the centerline of the innermostcore.

For example, optical fiber 600, shown in cross-sectional schematic viewin FIG. 6, has a innermost core 610 that includes a substantiallydown-doped region 615 disposed offset from the centerline of theinnermost core of the optical fiber (in this embodiment, also disposedoffset from the centerline of the optical fiber itself). As anotherembodiment, FIG. 7 is a cross-sectional schematic view of a mode mixingoptical fiber. The mode mixing optical fiber 700 includes an innermostcore 710, which includes a down-doped ring-shaped region 715; here, too,the down-doped region is disposed with its center offset from the centerof the innermost core (and also the centerline of the fiber). FIG. 8 isa cross-sectional schematic view of yet another embodiment of a modemixing optical fiber as described herein. Referring to FIG. 8, modemixing optical fiber 800 includes an innermost core 810, which includesdown-doped regions 815 a, 815 b, 815 c and 815 d. Here, the down-dopedregions 815 a, 815 b, 815 c and 815 d are disposed with their centers atvarious distances away from the centerline 804 of the optical fiber.When multiple down-doped regions are provided, they may be disposedrandomly, or alternatively, arranged in a regular geometricalarrangement (i.e., without forming a bandgap structure).

In certain embodiments, the innermost core of a mode mixing opticalfiber includes one or more substantially up-doped regions. For example,the one or more substantially up-doped regions may be disposedasymmetrically around the centerline of the innermost core, e.g., in anyof the manners described above with respect to the down-doped regions.In other embodiments, an up-doped region is disposed symmetricallyaround the center of the innermost core (e.g., as a ring-shaped region),but substantially towards the periphery of the innermost core (e.g.,with at least ½, ⅔ or even ¾ of the area of the up-doped area disposedat least half the radius of the innermost core away from the centerlineof the innermost core). If present, the one or more high-index regionscan be provided in combination with one or more low-index regions asdescribed above.

The person of ordinary skill in the art will appreciate that any numberof substantially up-doped/down-doped regions can be provided in theinnermost core of the optical fiber. For example, in certainembodiments, e.g., as described herein with respect to FIGS. 5, 6, 7, 9and 23, there is only a single substantially up-doped/down-doped regionin the innermost core. In other embodiments, for example, as describedwith respect to FIG. 8, there are a plurality of up-doped/down-dopedregions. However, for the sake of simplicity of manufacture, it can bedesirable to limit the number of up-doped/down-doped regions, e.g., tobe no more than 12, no more than 8, no more than 5, no more than 4, oreven no more than 3. When there are a plurality of up-doped/down-dopedregions, they do not form a so-called photonic crystal or photonicbandgap structure having a bandgap at the wavelength.

The innermost core of the mode mixing optical fiber can be formed in avariety of sizes. For example, in certain embodiments, the innermostcore of the optical fiber has a diameter (i.e., the radially-averageddistance across the innermost core) in the range of about 50 μm to about3000 μm, e.g., in the range of about 50 μm to about 2000 μm, or about 50μm to about 1000 μm, or about 50 μm to about 600 μm, or about 100 μm toabout 3000 μm, or about 100 μm to about 2000 μm, or about 100 μm toabout 1000 μm, or about 100 μm to about 600 μm, or about 200 μm to about3000 μm, or about 200 μm to about 2000 μm, or about 200 μm to about 1000μm, or about 200 μm to about 600 μm. The person of ordinary skill in theart will select an innermost core diameter to provide a desired numberof modes and overlap with an input optical fiber.

Similarly, the overall mode mixing optical fiber may be formed in avariety of sizes. In certain embodiments, the mode mixing optical has anouter diameter in the range of about 100 μm to about 3600 μm, e.g., inthe range of about 100 μm to about 3000 μm, or about 100 μm to about2500 μm, or about 100 μm to about 1500 μm, or about 100 μm to about 1000μm, or about 100 μm to about 800 μm, or about 100 μm to about 600 μm, orabout 200 μm to about 3600 μm, or about 200 μm to about 3000 μm, orabout 200 μm to about 2500 μm, or about 200 μm to about 1500 μm, orabout 200 μm to about 1000 μm, or about 200 μm to about 800 μm, or about200 μm to about 600 μm. In certain embodiments, the outer diameter ofthe mode mixing optical fiber is at least about 1.05 times the outerdiameter of the innermost core, for example, in the range of about 1.05to about 5 times, or about 1.05 to about 3 times, or about 1.05 to about2 times the outer diameter of the innermost core of the mode mixingoptical fiber. For example, in some embodiments, the outer diameter ofthe mode mixing optical fiber is at least about 1.2 times the outerdiameter of the innermost core, for example, in the range of about 1.2to about 5 times, or about 1.2 to about 3 times, or about 1.2 to about 2times the outer diameter of the innermost core of the mode mixingoptical fiber.

As the person of ordinary skill in the art will appreciate, the variousup-doped and down-doped regions may be provided in a variety of shapesand a variety of sizes. In certain embodiments, the various up-dopedand/or down-doped regions have a solid cross-sectional shape selectedfrom circular, non-circular but rounded (e.g., oval, ellipse,hemicircular), polygonal (e.g., triangular, hexagonal, square). Thevarious up-doped and/or down-doped regions can also be provided asannular shapes (e.g., circular rings, annular non-circular but roundedshapes, or annular polygons). The various up-doped and/or down-dopedregions can be, for example, at least about the size of the wavelength(i.e., in radially-averaged cross-sectional width). In certainembodiments, the various up-doped and/or down-doped regions are, forexample in the range of about 1 μm to about 2000 μm in size, e.g., inthe range of about 1 μm to about 1500 μm, or about 1 μm to about 1000μm, or about 1 μm to about 800 μm, or about 1 μm to about 600 μm, orabout 1 μm to about 400 μm, or about 1 μm to about 200 μm, or about 2 μmto about 2000 μm, or about 2 μm to about 1500 μm, or about 2 μm to about1000 μm, or about 2 μm to about 800 μm, or about 2 μm to about 600 μm,or about 2 μm to about 400 μm, or about 2 μm to about 200 μm, or about 5μm to about 2000 μm, or about 5 μm to about 1500 μm, or about 5 μm toabout 1000 μm, or about 5 μm to about 800 μm, or about 5 μm to about 600μm, or about 5 μm to about 400 μm, or about 5 μm to about 200 μm, orabout 15 μm to about 2000 μm, or about 15 μm to about 1500 μm, or about15 μm to about 1000 μm, or about 15 μm to about 800 μm, or about 15 μmto about 600 μm, or about 15 μm to about 400 μm, or about 15 μm to about200 μm. In certain embodiments, the total percentage of innermost corearea of the up- and/or down-doped regions is in the range of about 5% toabout 95%, e.g., in the range of about 5% to about 85%, or about 5% toabout 75%, or about 5% to about 50%, or about 5% to about 25%, or about10% to about 95%, or about 10% to about 85%, or about 10% to about 75%,or about 10% to about 50%, or about 10% to about 25%, or about 1% toabout 10%, or about 1% to about 20%, or about 1% to about 25%.

The one or more substantially up-doped and/or down-doped regions have arefractive index that is substantially different from that of theremainder of the innermost core. For example, the substantially-up dopedregions can have a refractive index (i.e., at the wavelength) that is atleast about 0.001, at least about 0.002, at least about 0.003, or evenat least about 0.005 greater than that of the remainder of the innermostcore, e.g., at least about 0.01 or at least about 0.02 greater than thatof the remainder of the innermost core. Similarly, the substantiallydown-doped regions can have a refractive index that is at least about0.001, at least about 0.002, at least about 0.003, or even at leastabout 0.005 less than that of the remainder of the innermost core, e.g.,at least about 0.01 or at least about 0.02 less than that of theremainder of the innermost core. However, in certain embodiments, theabsolute refractive index difference (i.e., at the wavelength) betweeneach up-doped/down-doped region and the remainder of the innermost coreis at most about 0.2, at most about 0.1, or at most about 0.05. Suchmaterials can be made from glasses similar to those of the remainder ofthe remainder of the innermost core, and thus can desirably havethermomechanical properties similar to those of the remainder of theinnermost core, simplifying manufacture. In certain desirableembodiments, the substantially up-doped and/or down-doped regions areformed as regions of index discontinuity within the innermost core(i.e., with the change in refractive index occurring within about 1 μmin linear distance along the cross-section of the innermost core).

In certain particular embodiments, the innermost core of the mode mixingoptical fiber includes a single annular shaped down-doped region, e.g.,in the shape of a circular ring. The annular shaped down-doped regioncan, for example, have an inner diameter in the range of 5 microns to 20microns, and an annular thickness in the range of 0.5 microns to 3microns. The refractive index of the down-doped region can be such thatthe relative numerical aperture value with respect to the remainder ofthe innermost core is, for example, in the range of 0.01 to 0.15, e.g.,0.01 to 0.10, 0.01 to 0.05, 0.02 to 0.10, or 0.02 to 0.05. As usedherein, the “diameter” of a non-circular feature is twice theradially-averaged distance from the geometrical center of the feature.

The mode mixing optical fiber can be provided in a variety of lengths.The person of ordinary skill in the art can select a length sufficientto provide the desired distribution of radiation into higher ordermodes. For example, in certain embodiments, the mode mixing opticalfiber has a length in the range of about 1 m to about 100 m, e.g., inthe range of about 1 m to about 50 m, or about 1 m to about 40 m, orabout 1 m to about 50 m, or about 1 m to about 20 m, or about 1 m toabout 10 m, or about 1 m to about 5 m, or about 5 m to about 100 m, orabout 5 m to about 100 m, e.g., in the range of about 5 m to about 50 m,or about 5 m to about 40 m, or about 5 m to about 50 m, or about 5 m toabout 20 m, or about 10 m to about 100 m, or about 10 m to about 50 m,or about 10 m to about 40 m.

In certain embodiments (including the embodiments of FIGS. 3, 4 and 6-8as described above), the mode mixing optical fiber does not have acircularly-symmetric cross-sectional profile. In certain suchembodiments, the cross-sectional profile of the mode mixing opticalfiber is formed as a helix along the length hereof. That is, in a statethat is not twisted by some external force, the various elements of theoptical fiber twist in a helical configuration along the length of thefiber, for example, with a pitch in the range of about 1 mm to about 100cm, e.g., in the range of about 1 mm to about 50 cm, or about 1 mm toabout 30 cm, or about 1 mm to about 20 cm, or about 1 mm to about 10 cm,or about 1 mm to about 5 cm, or about 2 mm to about 100 cm, or about 2mm to about 50 cm, or about 2 mm to about 30 cm, or about 2 mm to about20 cm, or about 2 mm to about 10 cm, or about 2 mm to about 5 cm, orabout 5 mm to about 100 cm, or about 5 mm to about 50 cm, or about 5 mmto about 30 cm, or about 5 mm to about 20 cm, or about 5 mm to about 10cm, or about 5 mm to about 5 cm, or about 1 cm to about 100 cm, or about1 cm to about 50 cm, or about 1 cm to about 30 cm, or about 1 cm toabout 20 cm, or about 1 cm to about 10 cm, or about 1 cm to about 5 cm.Such a configuration is shown in schematic view in FIG. 9. A section ofoptical fiber 900 having an off-center innermost core is shown in sideview, with the centerline 914 of the innermost core shown as a dashedline. The cross-sectional profile at each of positions A, B and C isshown. Notably, the off-center innermost core is formed as a helixthroughout the fiber. The efficiency of the mode mixing process can besignificantly increased by use of such a helical configuration. Such afiber may be made using conventional methodologies (e.g., by spinningthe preform during the draw of the optical fiber).

The mode mixing optical fiber can be made from conventional materialusing conventional methods in the art. For example, the optical fibercan be made using various silica-based glasses (e.g., germanosilicates,borosilicates, aluminosilicates, fluorosilicates and combinationsthereof). In certain embodiments, the innermost core (e.g., exclusive ofany up-doped or down-doped regions) is formed from substantially undopedsilica, while the cladding (at least in the region immediatelysurrounding the innermost core) includes fluorine-doped silica. In otherembodiments, the innermost core (e.g., exclusive of any down-dopedregions) is formed from germanium-doped silica, while the cladding (atleast in the region immediately surrounding the innermost core) includessubstantially undoped silica. Conventional dopants can be used toprovide up-doped and down-doped regions. Conventional methods of makingoptical fibers (e.g., stacking together various rods and tubes ofdifferent refractive indices, followed by collapsing them to a preformand drawing the preform) can be used to make the mode mixing opticalfibers described herein.

Based on the present disclosure, the person of ordinary skill in the artcan provide mode mixing optical fibers providing a wide variety of beamparameter products, and thus a wide variety of divergence angles. Forexample, in certain embodiments, the beam divergence of the mode mixingoptical is in the range of about 40 mrad, 60 mrad or 80 mrad up to thenumerical aperture of the optical fiber, e.g., in the range of about 40mrad to about 600 mrad, or about 40 mrad to about 300 mrad, or about 40mrad to about 160 mrad, or about 40 mrad to about 140 mrad, or about 40mrad to about 120 mrad, or about 40 mrad to about 100 mrad, or about 40mrad to about 80 mrad, or about 60 mrad to about 600 mrad, or about 60mrad to about 300 mrad, or about 60 mrad to about 160 mrad, or about 60mrad to about 140 mrad, or about 60 mrad to about 120 mrad, or about 60mrad to about 100 mrad, or about 80 mrad to about 600 mrad, or about 80mrad to about 300 mrad, or about 80 mrad to about 160 mrad, or about 80mrad to about 140 mrad, or about 80 mrad to about 120 mrad, or about 80mrad to about 100 mrad, or about 100 mrad to about 200 mrad, or about100 mrad to about 400 mrad, or about 100 mrad to about 600 mrad, orabout 200 mrad to about 600 mrad. Of course, the person of ordinaryskill in the art can provide mode mixing optical fibers having differentdivergence angles for different applications. For example, the person ofordinary skill in the art, in some embodiments, can provide mode mixingoptical fibers with a beam divergence angles as high as the NA of theinnermost core.

Similarly, based on the present disclosure, the person of ordinary skillin the art can provide mode mixing optical fibers providing asubstantially flat-top output. For example, the mode mixing opticalfiber can be configured to guide or output a beam (i.e., as defined byan outer periphery at 5% of the peak intensity) having at least 70%, atleast 80%, or even at least 90% of its cross-sectional area within about20%, within about 15%, or even within about 10% of its averageintensity. For example, the mode mixing fiber can be configured to guideor output radiation (i.e., as defined by an outer periphery at 5% of thepeak intensity) having at least 70%, at least 80%, or even at least 90%of its cross-sectional area within about 20%, within about 15%, or evenwithin about 10% of its average intensity, e.g., when the radiationinput to the mode mixing fiber has no more than 50%, no more than 40%,no more than 30% or even no more than 20% of its cross-sectional areawithin about 20%, within about 15%, or even within about 10% of itsaverage intensity (e.g., with its highest intensity at the center).

The mode mixing optical fibers can be provided with a variety ofnumerical aperture values. For example, in certain embodiments, thenumerical aperture of a mode mixing optical fiber is in the range ofabout 0.10 to about 0.60, e.g., in the range of about 0.10 to about 0.40or about 0.10 to about 0.30, or about 0.10 to about 0.22, or about 0.15to about 0.60, or about 0.15 to about 0.40, or about 0.15 to about 0.30.

The various regions of the refractive index profile of the innermostcore can perturb the propagation of radiation therein, e.g., by actingas a scattering (or weakly guiding) center, reflecting (or guiding) thelight away and populating the higher order modes of the innermost core.As the person of ordinary skill in the art will appreciate based on thepresent disclosure, the performance of the mode mixing optical fibersdescribed herein may be influenced by several design parametersincluding, for example, the lateral offset of the innermost core, therefractive index profile of the innermost core, the numerical aperture,the length of the mode mixing fiber, any coiling conditions (diameterand length), and any helicity of the refractive index profile of theinnermost core. The design of the mode mixing optical fiber may bescaled in order to provide a desired innermost core size (e.g., to matchthe size of a separate beam delivery fiber when one is used.

Another aspect of the disclosure is an optical system that includes amode mixing optical fiber as described above, and a first optical fiberhaving an output end directly optically coupled to the input end of themode mixing optical fiber, the first optical fiber being configured topropagate optical radiation having the wavelength. One such embodimentis shown in schematic side view in FIG. 10. Optical system 1030 includesa mode mixing optical fiber 1000, having an input end 1002 and an outputend 1004, as well as a first optical fiber 1040, having an output end1044. The output end 1044 of the first optical fiber is directlyoptically coupled to the input end 1002 of the mode mixing optical fiber(i.e., without any substantial optical component therebetween). Forexample, the output end of the first optical fiber can be fusion splicedto the input end of the mode mixing optical fiber. The first opticalfiber can couple light radiation to the input end of the mode mixingoptical fiber such that their centerlines are aligned with one another(i.e., even though the innermost core of the mode mixing optical fibermay be offset from the centerline of the mode mixing optical fiber).

Notably, the mode mixing optical fiber can accept radiation from asingle or few-moded optical fiber and, through distribution of radiationinto higher order modes, provide an output beam having desired opticalcharacteristics (e.g., as described above). Thus, in certainembodiments, the first optical fiber is single-mode at the wavelength.In other embodiments, the first optical fiber has 7 or fewer, 6 orfewer, 5 or fewer or even 4 or fewer modes at the wavelength. Of course,in other embodiments, the mode mixing fiber can accept radiation from amultimode optical fiber, or from a solid state source (e.g., viacoupling through free-space optics).

In certain advantageous embodiments, the first optical fiber isconfigured to provide radiation from an optical fiber laser or opticalfiber amplifier. For example, the first optical fiber can be an activeoptical fiber of a fiber laser or a fiber amplifier, e.g., a rare-earthdoped fiber, or a fiber configured to provide gain through somenon-linear process (e.g., Raman scattering, Brillouin scattering).

In certain embodiments, the first optical fiber has substantially thesame diameter as the mode mixing optical fiber. Such embodiments may beespecially advantaged, in that the alignment of the first optical fiberto the mode mixing optical fiber (i.e., for optical coupling, forexample, via fusion splicing) can be simplified. Similarly, in certainembodiments, the diameter of the innermost core of the first opticalfiber is within 10%, or even within 5% of the diameter of the innermostcore of the mode mixing optical fiber.

In certain embodiments, the mode mixing optical fiber can provide a beamhaving desirable optical characteristics from its second end. Forexample, in certain embodiments, the optical system is configured tolaunch a free space-propagating beam (e.g., as identified by referencenumeral 1060 in FIG. 10) from the second end of the mode mixing opticalfiber. In such embodiments, the mode mixing optical fiber can act as abeam delivery fiber, and can be configured in a beam delivery cable,e.g., ruggedized to allow for handling in an industrial environment. Ifnecessary, additional optics (e.g., collimating lenses and/or otherdiffractive or refractive elements) can be provided at the output end ofthe mode mixing optical fiber.

In other embodiments, the optical system further includes a secondoptical fiber, the second optical fiber being a multi-mode at thewavelength, the second optical fiber having an input end and an outputend, the input end of the second optical fiber being directly opticallycoupled to the output end of the mode mixing optical fiber. Oneparticular embodiment is shown in schematic view in FIG. 11. Opticalsystem 1130 includes a first optical fiber 1140 and a mode mixingoptical fiber 1100 with the output 1144 of the first optical fiberdirectly optically coupled to the input 1102 of the mode mixing opticalfiber as described above. Optical system 1100 further includes a secondoptical fiber 1150, having an input end 1152 and an output end 1154,with the input end 1152 of the second optical fiber directly opticallycoupled (here, fusion spliced) to the output end 1104 of the mode mixingoptical fiber. In such embodiments, the mode mixing optical fiber canact to transform the optical characteristics of the output of the firstoptical fiber (e.g., intensity profile) to a more desirable state (e.g.,having a flat-top intensity profile) to be coupled into the secondoptical fiber.

The system can be configured to launch a free space-propagating beam(e.g., as identified by reference numeral 1160 in FIG. 11) from thesecond end of the second optical fiber. In such embodiments, the secondoptical fiber can act as a beam delivery fiber, and can be configured ina beam delivery cable, e.g., ruggedized to allow for handling in anindustrial environment. If necessary, additional optics (e.g.,collimating lenses and/or other diffractive or refractive elements) canbe provided at the output end of the second optical fiber.

In other embodiments, an optical system includes a mode mixing opticalfiber having its input end coupled to the output of an optical source,such as a solid state laser. The optical source can be coupled to theinput end of the mode mixing fiber, for example, using free-spaceoptics. An example of such an embodiment is shown in FIG. 12. Opticalfiber system 1230 includes an optical source 1270 (e.g., a solid-statelaser) having its output coupled to the input end 1202 of mode mixingoptical fiber 1200 through free-space optics 1275 (e.g., one or morelenses). A free space-propagating beam (e.g., as identified by referencenumeral 1260 in FIG. 12) can be emitted from the second end 1200 of themode mixing optical fiber.

The diameter of the innermost core of the second optical fiber can varydepending on the end-user needs, for example, to allow forimplementation in already existing systems. The diameter of theinnermost core of the second optical fiber can be, for example, withinabout 10%, or even within about 5% of the diameter of the innermost coreof the mode mixing optical fiber. Of course, in other embodiments, theinnermost core of the second optical fiber can be a different size,e.g., in the range of about 50 μm to about 3000 or about 50 μm to about2000 or about 50 μm to about 1000 or about 50 μm to about 600 or about100 μm to about 3000 or about 100 μm to about 2000 or about 100 μm toabout 1000 or about 100 μm to about 600 or about 200 μm to about 3000 orabout 200 μm to about 2000 or about 200 μm to about 1000 or about 200 μmto about 600 μm.

The systems described herein can be configured to output a beam (i.e.,as defined by an outer periphery at 5% of the peak intensity) having atleast 70%, at least 80%, or even at least 90% of its cross-sectionalarea within about 20%, within about 15%, or even within about 10% of itsaverage intensity, e.g., when the radiation input to the mode mixingfiber has no more than 50%, no more than 40%, no more than 30% or evenno more than 20% of its cross-sectional area within about 20%, withinabout 15%, or even within about 10% of its average intensity (e.g., withits highest intensity at the center).

Another aspect of the disclosure is a method for providing guidedradiation of the wavelength having a desired intensity profile. Themethod includes coupling input radiation into a first end of a modemixing fiber as described herein, and guiding the radiation along themode mixing optical fiber to provide guided radiation having a desiredintensity profile, e.g., a flat-top intensity profile as describedherein. In certain embodiments, the guided radiation (i.e., as definedby an outer periphery at 5% of the peak intensity) having the desiredintensity profile has at least 70%, at least 80%, or even at least 90%of its cross-sectional area within about 20%, within about 15%, or evenwithin about 10% of its average intensity. The radiation can be guidedalong a length of the mode mixing optical fiber of, for example, in therange of about 1 m to about 100 m, e.g., in the range of about 1 m toabout 50 m, or about 1 m to about 40 m, or about 1 m to about 50 m, orabout 1 m to about 20 m, or about 1 m to about 10 m, or about 1 m toabout 5 m, or about 5 m to about 100 m, or about 5 m to about 100 m,e.g., in the range of about 5 m to about 50 m, or about 5 m to about 40m, or about 5 m to about 50 m, or about 5 m to about 20 m, or about 10 mto about 100 m, or about 10 m to about 50 m, or about 10 m to about 40 mto provide the radiation having the desired intensity profile. Incertain embodiments, the input radiation has a substantially differentintensity profile than the desired intensity profile. For example, incertain embodiments, the input radiation has no more than 50%, no morethan 40%, no more than 30% or even no more than 20% of itscross-sectional area within about 20%, within about 15%, or even withinabout 10% of its average intensity (e.g., with its highest intensity atthe center). The method can be used in conjunction with any of the modemixing fibers or systems as described herein.

Another aspect of the disclosure is a method for providing a freespace-propagating optical beam using an optical system as describedherein. The method includes propagating radiation of the wavelength fromthe first optical fiber into the mode mixing optical fiber; andpropagating the radiation from the output end of the mode mixing opticalfiber. If the system includes a second optical fiber as described above,the method can further include propagating the radiation through thesecond optical fiber and from its output end. The method can beperformed such that the divergence, BPP and/or flatness is as describedin any embodiment above. For example, in certain embodiments, the beam(i.e., as defined by an outer periphery at 5% of the peak intensity)having the desired intensity profile has at least 70%, at least 80%, oreven at least 90% of its cross-sectional area within about 20%, withinabout 15%, or even within about 10% of its average intensity. In certainembodiments, the input radiation has a substantially different intensityprofile than the desired intensity profile. For example, in certainembodiments, the input radiation has no more than 50%, no more than 40%,no more than 30% or even no more than 20% of its cross-sectional areawithin about 20%, within about 15%, or even within about 10% of itsaverage intensity (e.g., with its highest intensity at the center).

Various aspects and embodiments of the disclosure will be furtherexplained with reference to the following non-limiting Examples:

EXAMPLES

The mode mixing effect of certain mode mixing optical fibers describedherein was demonstrated both numerically and experimentally.

Example 1

In Example 1, the overall test configuration was as shown in FIG. 11,with the mode mixing fiber configured to transform radiation output froma large mode area single mode fiber to a beam delivery cable.

The mode mixing optical fiber is shown in schematic view in FIG. 13,with the cleaved fiber endface shown in the photograph of FIG. 14. Themode mixing optical fiber has a germanium-coped core 60 μm in diameter,with a step index profile. The core has a numerical aperture of 0.11,and is laterally offset with respect to the centerline of the overalloptical fiber by 20 μm. The overall fiber diameter is 360 μm.

The first optical fiber is a conventional large mode area single modefiber having a 20 μm diameter core, numerical aperture of 0.06, andoverall diameter of 400 μm. The second optical fiber (i.e., of the beamdelivery cable) is matched to certain commercially available devices,and has a 100 μm diameter core, a numerical aperture of 0.22, an overallfiber diameter of 360 μm and a length of 25 m.

The results of calculations are shown in FIGS. 15-17. The powerdistribution among the modes excited in the beam delivery cable is shownin FIG. 15. The total output intensity delivered by the beam deliverycable is shown in FIG. 16, and the corresponding beam profile is shownin FIG. 17. The output beam is flat-top shaped and the BPP is estimatedto be about 3.4 mm·mrad.

The mode mixing effect induced by the mode mixing fiber appears clearlywhen comparing these results to the case without a mode mixing fiber,shown in FIGS. 18-20. With the otherwise same parameters, the simulationperformed without the mode mixing optical fiber provides asharply-peaked output beam,

Experimental results were also collected. In the case without the modemixing optical fiber (as described with respect to FIGS. 18-20), thebeam emerging the conventional beam delivery cable was characterized byrecording the near-field intensity profile and the BPP. Results areshown in FIG. 21. Due to the low degree of mode-mixing, the beam profileis very uneven and the measured BPP of 2.5 mm·mrad out of the especiallydesired range of 3 to 4 mm·mrad. In contrast, when using the mode mixingoptical fiber, as described above with respect to FIGS. 13-17, thenear-field profile shows a good uniformity (FIG. 22) with BPP valuesaround 3.5 mm·mrad.

Example 2

In this example, the system was configured with the mode mixing opticalfiber as a beam delivery fiber (e.g., as shown in FIG. 11). Here, too,both simulation and experimental results are presented. Here, the modemixing optical fiber has a silica core 100 μm in diameter, surrounded bya down-doped fluorine cladding layer sufficient to provide a numericalaperture of 0.22, with a silica outer cladding to provide an overallfiber diameter of 360 μm. The core includes a low-index ring formed byfluorine-doped silica. The ring is 4 μm in annular thickness, having aninner diameter of 30 μm with its center laterally offset from thecenterline of the optical fiber by 12 μm. The index contrast between thering and the remainder of the innermost core is sufficient to provide anumerical aperture value of 0.1 (i.e., between the material of the ringand that of the rest of the innermost core; the ring itself is notsufficient to guide light of the wavelength). The design is shown inschematic cross-sectional view in FIG. 23. The first optical fiber is aconventional large mode area single mode fiber as described above inExample 1. Calculation results are summarized in FIGS. 25-27, in whichthe power distribution among the modes excited in the mode mixingoptical fiber is shown in FIG. 25; the total output intensity deliveredby the second end of the mode mixing optical fiber is shown in FIG. 26;and the corresponding beam profile is shown in FIG. 27.

As noted above, in this Example, the mode mixing fiber is configured asa beam delivery cable. The mode up-conversion is demonstrated on theplot showing the power distribution in FIG. 25 (only the first 100 modeswere plotted for clarity purposes). With these exact parameters, the BPPwas estimated around 4 mm·mrad. However, the output beam is notcalculated to be exactly flat-top shaped (although it is remarkably flatas compared to a Gaussian beam). This can be changed by the person ofordinary skill in the art by modifying appropriately the design of thefiber, the size and location of the core elements.

An experimental demonstration of the mode mixing created by theup-conversion beam delivery cable using the mode mixing fiber of FIGS.23 and 24 is shown in FIGS. 28-30. The measurement setup is shown inschematic view in FIG. 28. The measured intensity and beam profiledisplayed respectively in FIGS. 29 and 30 demonstrate good uniformitywith a BPP measured at 3.9 mm·mrad, demonstrating the mode up-conversionoccurring in this fiber used as beam delivery cable. This can becompared to the results shown in FIG. 21, which resulted from the use ofa conventional beam delivery cable. Here, the low-index ring does appearin the output intensity profile with a 50% intensity contrast comparedto the maximum intensity. This ring contrast can be reduced by modifyingappropriately the fiber design, as shown below with respect to Example3.

Example 3

In this example, the system was configured substantially similarly tothe configuration of Example 2, but with the numerical aperture value ofthe low-index ring in the core being 0.02 (i.e., instead of 0.1 as inExample 2).

An experimental demonstration of the mode mixing created by theup-conversion beam delivery cable using the mode mixing fiber of thisExample is shown in FIGS. 31 and 32. The measured intensity and beamprofile displayed respectively in FIGS. 31 and 32 demonstrate gooduniformity. This can be compared to the results Example 2, demonstratingthat in fact appropriate design can maintain a desirably flat-topprofile while reducing the intensity contrast caused by the down-dopedring.

Example 4

In this example, mode mixing optical fibers of different core diameterswere used, one of 50 μm core and another of 25 μm core diameter, withsingle mode input radiation. A first experiment using a 50 μm core modemixing fiber is described with respect to FIG. 33. Here, an LMA/GDF20/400 fiber was spliced to the input end of a 25 m long mode mixingoptical fiber having a down-doped ring disposed offset within the innercore; details are shown in FIG. 33. Single mode radiation at wavelength1.06 μm was coupled from the LMA-GDF 20/400 fiber into the mode mixingoptical fiber. The intensity was measured at cleaved endface at theoutput of the mode mixing optical fiber. As shown in FIG. 33, the outputintensity was substantially uniform, but for some intensity contrast atthe position of the ring-shaped down-doped element. The BPP was about1.3 mm-mrad. In a second experiment, the mode mixing optical fiber wassimilar to that of the first experiment above, but with a 25 μm corediameter; dimensions are provided in FIG. 33. The mode mixing fiber was5 m long and was coiled with an 8 mm coil diameter. Here, the inputfiber was a single mode SMF-28 fiber, and the radiation was ofwavelength 1.55 m. Single mode radiation was coupled from the LMA-GDF20/400 fiber into the mode mixing optical fiber. Here, too, as shown inFIG. 34, the output intensity was substantially uniform. The BPP wasabout 1.5 mm-mrad.

Example 5

This example demonstrates the desirable increase in beam divergence fora mode mixing optical fiber having a helical profile. Two mode mixingoptical fibers were made, identical but for the fact that for the “spun”fiber, the preform was spun during drawing to provide a helical profilewith a period of 50 revolutions/m. Beam divergence was measured as afunction of fiber length using a cutback methodology. Results are shownin the graph of FIG. 35. Notably, beam divergence was substantiallyincreased for the spun fiber.

Example 6

In this example, various multimode fibers were spliced to a mode mixingfiber as described herein. In each case, the mode mixing fiber was 25 mlong, and was otherwise similar to the mode mixing fiber of Example 3,but for the NA value of the down-doped ring being 0.05. The 2D beamintensities of the input multimode fiber and the output of the modemixing fiber are provided in FIG. 36, and demonstrate that relativelyinhomogeneous intensities in the input multimode fiber were converted torelatively flat-top intensities by the mode mixing fibers. The graph ofFIG. 36 demonstrates that there is relatively little impact on beambrightness.

In the claims as well as in the specification above all transitionalphrases such as “comprising”, “including”, “carrying”, “having”,“containing”, “involving”, and the like are understood to be open-ended.Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases.

It is understood that the use of the term “a”, “an” or “one” herein,including in the appended claims, is open ended and means “at least one”or “one or more”, unless expressly defined otherwise. The occasional useof the terms herein “at least one” or “one or more” to improve clarityand to remind of the open nature of “one” or similar terms shall not betaken to imply that the use of the terms “a”, “an” or “one” alone inother instance herein is closed and hence limited to the singular.Similarly, the use of “a part of”, “at least a part of” or similarphrases (e.g., “at least a portion of”) shall not be taken to mean thatthe absence of such a phrase elsewhere is somehow limiting.

Subsequently reference to the phrase “at least one”, such as in thephrase “said at least one”, to specify, for example, an attribute of thelimitation to which “at least one” initially referred is not to beinterpreted as requiring that the specification must apply to each andevery instance of the limitation, should more than one be underconsideration in determining whether the claim reads on an article,composition, machine or process, unless it is specifically recited inthe claim that the further specification so applies.

The use of “or”, as in “A or B”, shall not be read as an “exclusive or”logic relationship that excludes from its purview the combination of Aand B. Rather, “or” is intended to be open, and include all permutation,including, for example A without B; B without A, and A and B together,and as any other open recitation, does not exclude other features inaddition to A and B.

Any of the features described above in conjunction with any one respectdescribed above can be combined with a practice of the inventionaccording to any other of the aspects described above, as is evident toone of ordinary skill who studies the disclosure herein.

Those of ordinary skill in the art will recognize or be able toascertain using no more than routine experimentation many equivalents tothe specific embodiments of the invention described herein. It istherefore to be understood that the foregoing embodiments are presentedby way of example only and that within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described. The present invention is directed to eachindividual feature, system, material and/or method described herein. Inaddition, any combination of two or more such features, systems,materials and/or methods, if such features, systems, materials and/ormethods are not expressly taught as mutually inconsistent, is includedwithin the scope of the present invention.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopebeing indicated by the following claims, along with the full scope ofequivalents to which such claims are entitled. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

What is claimed is:
 1. A mode mixing optical fiber for deliveringoptical radiation having a wavelength, the mode mixing optical fiberhaving a input end, an output end, a centerline and a refractive indexprofile, the mode mixing optical fiber comprising: an innermost core foroptical radiation having the wavelength, the innermost core having asubstantially circular cross-sectional shape, the innermost core havinga refractive index profile and a centerline, the centerline of theinnermost core being positioned substantially collinearly with thecenterline of the mode mixing optical fiber; and a cladding disposedabout the innermost core, wherein the refractive index profile of theinnermost core comprises one or more substantially doped regionsdisposed asymmetrically with respect to the centerline of the innermostcore, each of the substantially doped regions being substantiallyup-doped or substantially down-doped, wherein the mode mixing opticalfiber has at least thirty modes at the wavelength, wherein therefractive index profile of the innermost core is configured todistribute a fraction of the light input to the optical fiber at itsinput end from its lower-order modes to its higher-order modes as thelight propagates toward the output end.
 2. The mode mixing optical fiberaccording to claim 1, wherein there is no substantially down-dopedregion disposed symmetrically around the centerline of the innermostcore.
 3. The mode mixing optical fiber according to claim 1, wherein theone or more substantially doped regions includes one or moresubstantially down-doped regions.
 4. The mode mixing optical fiberaccording to claim 1 wherein the one or more substantially doped regionsdisposed asymmetrically with respect to the centerline of the innermostcore includes a singular annular shaped down-doped region, wherein thesingular annular shaped down-doped region does not encompass thecenterline of the innermost core.
 5. The mode mixing optical fiberaccording to claim 4, wherein the annular shaped down-doped region is inthe shape of a circular ring.
 6. The mode mixing optical fiber accordingclaim 4, wherein the annular shaped down-doped region has an innerdiameter in the range of 5 microns to 20 microns and an annularthickness in the range of 0.5 microns to 3 microns.
 7. The mode mixingoptical fiber according to claim 4, wherein the refractive index of theannular shaped down-doped region is such that the relative numericalaperture value with respect to the remainder of the innermost core is inthe range of 0.01 to 0.15.
 8. The mode mixing optical fiber according toclaim 1, wherein the one or more substantially doped regions includesone or more substantially up-doped regions.
 9. The mode mixing opticalfiber according to claim 8, wherein the cross-sectional profile of themode mixing optical fiber is formed as a helix along the length thereof.10. The mode mixing optical fiber according to claim 1, wherein theoptical fiber has a length in the range of about 1 m to about 100 m. 11.The mode mixing optical fiber according to claim 1, in which thedivergence of the mode mixing optical fiber is in the range of about 40mrad to the numerical aperture of the innermost core.
 12. The modemixing optical fiber according to claim 1, configured to output a beam,as defined by an outer periphery at 5% of the peak intensity, having atleast 80% of its cross-sectional area within about 15% of its averageintensity, when the beam of radiation input to the mode mixing fiber, asdefined by an outer periphery at 5% of the peak intensity, has itshighest intensity at its center and no more than 50% of itscross-sectional within about 15% of its average intensity.
 13. Anoptical system comprising: a mode mixing optical fiber according toclaim 1, the mode mixing optical fiber being configured for deliveringoptical radiation having a wavelength; and a first optical fiber havingan output end directly optically coupled to the input end of the modemixing optical fiber, the first optical fiber being configured topropagate optical radiation having the wavelength.
 14. The opticalsystem according to claim 13, wherein the first optical fiber is anactive optical fiber of a fiber laser or fiber amplifier.
 15. An opticalsystem comprising: a mode mixing optical fiber according to claim 1; andan optical source optically coupled to the input end of the first modemixing optical fiber.
 16. A method for providing guided radiation of awavelength having a desired intensity profile, the method comprisingcoupling input radiation into a first end of a mode mixing fiberaccording to claim 1, and guiding the radiation along the mode mixingoptical fiber to provide guided radiation having an intensity profile.17. The method according to claim 16, wherein the input radiation, asdefined by an outer periphery at 5% of the peak intensity, has itshighest intensity at its center and has no more than 40% of itscross-sectional area within about 15% of its average intensity; and theguided radiation, as defined by an outer periphery at 5% of the peakintensity, has at least 80% of its cross-sectional area within about 15%of its average intensity.
 18. The mode mixing optical fiber according toclaim 1, wherein each of the one or more substantially doped regions hasa refractive index that is in the range of about 0.005 to about 0.1different than that of the remainder of the innermost core.
 19. A modemixing optical fiber for delivering optical radiation having awavelength, the mode mixing optical fiber having a input end, an outputend, a centerline and a refractive index profile, the mode mixingoptical fiber comprising: an innermost core for optical radiation havingthe wavelength, the innermost core having a substantially circularcross-sectional shape, the innermost core having a refractive indexprofile and a centerline, the centerline of the innermost core beingpositioned substantially collinearly with the centerline of the modemixing optical fiber; and a cladding disposed about the innermost core,wherein the refractive index profile of the innermost core comprises asingular annular shaped down-doped region that does not encompass thecenterline of the innermost core, and wherein the refractive index ofthe annular shaped down-doped region is such that the relative numericalaperture value with respect to the remainder of the innermost core is inthe range of 0.01 to 0.15, wherein the mode mixing optical fiber has atleast thirty modes at the wavelength, and wherein the refractive indexprofile of the innermost core is configured to distribute a fraction ofthe light input to the optical fiber at its input end from itslower-order modes to its higher-order modes as the light propagatestoward the output end.